1. Field of the Invention
The present invention relates to a liquid-crystal display apparatus and a method of driving the same liquid-crystal display apparatus. More particularly, the present invention relates to a liquid-crystal display apparatus which is capable of displaying a high-quality image and a method of driving the same liquid-crystal display apparatus.
2. Description of the Related Art
In recent years, liquid-crystal display apparatuses which can be formed into thin apparatuses as display elements and which use liquid-crystal display elements which consume a small amount of power have come to be increasingly practical.
An explanation will be given below of a color liquid-crystal display apparatus and a method of driving the same liquid-crystal display apparatus with reference to the drawings.
FIG. 1(a) is a schematic block diagram illustrating an example of a color liquid-crystal display apparatus, and FIG. 1(b) is a schematic view illustrating the color arrangement of a filter thereof. In FIGS. 1(a) and 1(b), reference numeral 10 denotes a liquid-crystal display element; reference numeral 11 denotes a switching transistor, such as a thin film transistor (TFT), in which amorphous silicon or polysilicon is used in a semiconductor layer; reference numeral 12 denotes a pixel electrode; reference numeral 13 denotes a row control line; reference numeral 14 denotes a column control line; reference numeral 20 denotes a vertical scanning circuit (V.multidot.SR); reference numeral 30 denotes a horizontal scanning circuit (H.multidot.SR); reference numeral 40 denotes a signal processing circuit; and reference numeral 50 denotes a control circuit. In a filter 15 shown in FIG. 1(b), R designates red, G designates green, and B designates blue. This filter 15 corresponds to the pixel electrode 12 in this order of color arrangement.
As shown in FIG. 1(a), the liquid-crystal display element 10 has switching transistors 11 for each pixel. The switching transistors have a great number of pixels such that the source (or drain) is connected to the column data line 14, the drain (or source) is connected to the pixel electrode 12, and the gate is connected to row control line 13. The pixel electrodes 12 are arranged in horizontal and vertical lines, and in correspondence with this arrangement, the colors in the filter 15 are arranged in horizontal and vertical lines.
The row control lines 13 are each connected to the vertical scanning circuit 20, and the column control lines 14 are each connected to the horizontal scanning circuit 30. A signal from the control circuit 50 is input to each of the vertical scanning circuit 20 and the horizontal scanning circuit 30. Further, a signal having image information is input from the signal processing circuit 40 to the horizontal scanning circuit 30.
Pulses are in turn applied from the vertical scanning circuit 20 to the row control lines 13 at every horizontal scanning period so that the switching on/off of the switching transistors 11 for the respective adjacent pixels is controlled. The color signals R, G and B from the signal processing circuit 40 are in turn selected by the horizontal scanning circuit 30 and supplied to the column control line 14. The control circuit 50 drives and controls the vertical scanning and horizontal scanning of the display apparatus, and the signal processing circuit in accordance with the operation of the system.
FIG. 2 shows a method of inputting color signals in the case of the color filter arrangement shown in FIGS. 1(a) and 1(b). In the color filter shown in FIGs. 1(a) and 1(b), it is necessary to input signals in the order of R, G and B for one pixel line when seen from the column data line 14. Therefore, the color signals of signal lines 31, 32 and 33 are switched by a color switching circuit 41 for each line.
Therefore, the signals having color information for each of R, G and B from the signal processing circuit 40 are distributed into signals having color information corresponding to each filter 15, and then input to the signal lines 31, 32 and 33. A switching element 16 is turned on/off by the horizontal scanning circuit 30, thereby supplying a signal having color information corresponding to the pixel connected to the column data line 14.
However, in the case of FIGS. 1(a) and 1(b), since the same color filters are arranged obliquely, the image is obliquely seen as a color and a line, and the image quality is deteriorated. Also, since a color switching circuit is necessary, it has been considered to prevent the image quality from being deteriorated and to construct the apparatus by using a small number of circuits.
An example of the above will be explained below with reference to FIG. 3. In the example shown in FIG. 3, to solve the problem of the above-described image deterioration, the odd-number and even-number columns of the pixel columns connected to the row control lines 13 are each repeated in the filter order of the same colors, and the repeat unit of the color filters arranged in the even-number columns is shifted by 11/2 pixels from the odd-number columns, i.e., a so-called delta arrangement.
In the column data line 14, pixels arranged in a staggered form are connected in units of the same colors. When this is done, the horizontal spacing frequency becomes twice improved and the resolution is improved when seen from the pixels in adjacent lines. Also, since the lines of the same colors are connected to the column electrode lines, the color switching circuit becomes unnecessary. Further, since the pixels of the same color are not arranged obliquely, the problem of the oblique color lines can be eliminated.
The arrangement shown in FIG. 3 as described above is used for a simplified electronic view finder (EVF) for field display, formed of about 230 pixels. In a field display of a display element which does not have such a high resolution as above, if the pixel sampling at every horizontal scanning is performed shifted by 11/2 pixels, it is possible to make an image display free from problems.
FIG. 4 is a block diagram illustrating another example of an active matrix type color liquid-crystal display apparatus. Reference numeral 410 denotes a display element section; reference numeral 420 denotes a vertical scanning circuit for vertically scanning the display element section 410; reference numeral 430 denotes a sampling circuit for sampling input image signals and outputting them to the display element section 410; and reference numeral 440 denotes a horizontal scanning circuit.
The unit pixel of the display element section 410 is formed of a switching transistor 411, a liquid crystal and a pixel holding capacitance 412. The gate of the switching transistor 411 is connected to the vertical scanning circuit 420 through a gate line 413, and the input terminal of the switching transistor 411 is connected to the sampling circuit 430 through a vertical data line 414. The other terminal of the pixel holding capacitance 412 is connected to a common electrode line 412-A, to which terminal a common electrode voltage V.sub.LC is applied.
Color signals (red, blue, green) are supplied from a signal processing circuit 450 to the input of the sampling circuit 430. The signal processing circuit 450 performs gamma processing in which liquid crystal characteristics are taken into consideration, inverted signal processing for making the liquid crystal have a longer service life, and other processing on input image signals. In a control circuit 460, necessary pulses are formed which are supplied to the vertical scanning circuit 420, the horizontal scanning circuit 440, the signal processing circuit 450, and the like.
FIG. 5 is an equivalent circuit diagram of the display element section 410 and the sampling circuit 430. Each line is formed in the display element section 410 in such a way that R, G and B pixels corresponding to the different three colors red, green and blue are repeatedly arranged horizontally in sequence in the order of R, G and B, and a plurality of pixel lines arranged vertically are provided therein. The pixel positions of the same colors are shifted by 1.5 pixels between the adjacent lines. That is, the pixels (R, G and B) are arranged in a delta form, and pixels of the same colors are connected to each data line 414 (d1, d2 . . . ) at every other line at both sides of the vertical data line 414. The sampling circuit 430 comprises switching transistors SW1, SW2 . . . , and capacitance (the parasitic capacitance and pixel capacitance of the vertical data lines). When the gates of the switching transistors SW1, SW2 . . . are driven by pulses h1, h2 . . . from the horizontal scanning circuit 440, respectively, the signal of each color of an input signal line 416 is transferred to each pixel through the data line 414 (d1, d2 . . . ) and written. The selection of a row at that time is controlled by vertical pulses .phi.g1 and .phi.g2 . . . from the vertical scanning circuit 420.
FIG. 6 is an illustration of an interlace scanning in a liquid-crystal display apparatus having the same number of vertical pixels as that of a television. The pixels of each row (hereinafter referred to as row pixels) in the display element section are made to correspond to the vertical pulses .phi.g1 and .phi.g2 . . . , and designated by symbols g1, g2 . . . . In the odd-number fields, the signal of the horizontal scanning line odd1 is written in row pixels g2 and g3, and similarly the signal of the horizontal scanning line odd2 is written in row pixels g4 and g5. The row pixels are driven in units of two rows for odd3 and subsequent scanning lines. In the even-number fields, the scanning combination is shifted by one line, and the signal of even is written in row pixels g3 and g4. Similarly, the subsequent signals are written in units of two rows.
An example of a drive timing in a case in which the scanning example of FIG. 6 is applied to the example of FIG. 4 is shown in FIG. 7 (this drive method is called a two-line simultaneous drive). In the scanning line odd1 in the odd-number field, the vertical pixels g2 and g3 corresponding to the row pixels g2 and g3 reach "H" (high state), causing each of the switching transistors 411 of that row pixel to conduct. Thus, the image signals sampled in sequence by the sampling circuit 430 are written in each pixel of row pixels g2 and g3. This sampling is performed in the "H" period of the horizontal scanning pulses h1, h2 . . . . The scanning of odd2 and subsequent scanning lines is similarly performed.
In recent years, there has been an increasing demand for a liquid-crystal display element used, in particular, in an EVF or a liquid-crystal projector to have a higher resolution image. In an EVF or a liquid-crystal projector, for example, a panel having vertical 460 pixels or more is under development to obtain a higher resolution image. When television signals are displayed on a panel having vertical 460 pixels, as described above, first an interlace drive is considered. When alternating inverted drive is performed at a frequency of 30 Hz in interlace drive, a flicker of 15 Hz is generated. To reduce this flicker, it is necessary to drive each pixel at 60 Hz, i.e., a field frequency.
Accordingly, when field drive is performed in the construction shown in FIG. 2, a method of simultaneously driving two rows of pixels as in the example described above is conceivable. Although flicker can be reduced by a two-line simultaneous drive, the horizontal resolution is deteriorated since the same sampling signal is applied to pixels shifted by 1.5 pixels between two rows.
According to the two-line simultaneous drive, since the same sampling signal is written in the pixel separated spatially by 1.5 pixels of the two rows of pixels which are driven simultaneously, the drive method is simple. However, the sampling frequency is not improved, and color moire occurs at a low resolution. Also, the pixel-shifted arrangement in which the pixels are shifted by 1.5 pixels horizontally exerts an adverse influence such that the edge of the image is displayed zigzag by the driving on the basis of the combination of row pixels shifted by one line between the odd-number fields and the even-number fields.
Since the pixels of three colors (R, G and B) are sampled in a point sequential manner by the horizontal scanning pulses h1, h2 and h3, the drive frequency becomes high to a greater extent in a panel having a great number of pixels. For example, in a panel having about 600 horizontal pixels in an NTSC system, the sampling frequency for two rows in which the pixel-shifted arrangement is taken into consideration becomes about 20 MHz. It is required in the Hi-Vision display that the number of horizontal pixels be 1,500 or more. In that case, the sampling frequency becomes about 50 MHz or more. Even in a current TFT liquid crystal, the drivable frequency is 10-odd MHz. Therefore, a plurality of scanning circuits are required to drive a panel having a great number of pixels.
In this way, the two-line simultaneous (field shifted) drive method described above could deteriorate the resolution. Also, since the horizontal drive frequency is increased, a plurality of scanning circuits are required, causing a problem, for example, that a great number of drive pulses are required, causing an increase in the consumed electric current.
Accordingly, column a electrode line connection as shown in FIG. 8 which does not cause the horizontal resolution to deteriorate is conceivable. FIG. 8 shows an arrangement in which the number of the column data lines 14 is double and the same-color pixels are connected together. With such an arrangement, and when the sampling of two rows of pixels is shifted at H.sub.1n and H.sub.2n, it is possible to eliminate the deterioration of the horizontal resolution.
However, an increase in the wiring of the column data lines causes the semiconductor process to be complex, and the aperture ratio of each pixel is greatly decreased. Therefore, when formation of such a fine structure is considered, the above construction cannot be said to be an appropriate one.
Also, a display method which displays a non-interlaced image by using a frame memory or a field memory is conceivable. Specifically, such a method entails a double-speed scanning in which the image signal is doubled and the frequency of the horizontal scanning is made twice as high and two horizontal row pixels are driven in sequence in one horizontal scanning period, as shown in FIG. 9.
An image improvement method of the above-described two-line simultaneous drive method includes such double-speed scanning. However, in the double-speed scanning, a frame memory and a high-band signal processing IC are required, a large amount of cost is incurred, and a large amount of power is consumed by the display apparatus.